A type of hybrid electric vehicle known as a series-type hybrid electric vehicle has been developed and put to practical use, in which a generator is driven by the power of an engine which is not used for moving the vehicle, the electric power generated by the generator is stored in a battery, and an electric motor is driven by the electric power supplied from the battery and used as a power source for moving the vehicle. In such a series-type hybrid electric vehicle, when the storage rate of the battery has become low, the engine is started to operate, and the electric power generated by the generator that is driven by the engine is stored in the battery. When the storage rate of the battery is recovered up to a predetermined storage rate by the electric power generated by the generator, the engine is stopped.
Compared with an engine used as a power source for moving a vehicle, the engine used in the series-type hybrid electric vehicle can be operated in an operation region where pollutants contained in the exhaust gas are relatively small. However, the exhaust gas from the engine still contains pollutants, and therefore, an exhaust purification device is used to purify the exhaust gas. An exhaust purification device for a hybrid electric vehicle has been proposed in Unexamined Japanese Patent Publication No. 2005-248875 (hereinafter, referred to as Patent Document 1), in which the temperature of a catalyst provided in the exhaust purification device is kept at a proper temperature to satisfactorily purify the exhaust gas.
The exhaust gas from the engine also contains NOx (nitrogen oxides) as a kind of pollutant. An exhaust purification device has been known in which an ammonia selective reduction-type NOx catalyst is disposed in the exhaust passage of the engine to reduce the NOx and thereby purify the exhaust gas. Thus, it is conceivable that such an ammonia selective reduction-type NOx catalyst may be applied to the exhaust purification device for a hybrid electric vehicle.
Where the ammonia selective reduction-type NOx catalyst is used in an exhaust purification device, generally, urea water, which is easier to handle than ammonia, is fed into the exhaust gas in order to supply ammonia as a reducing agent to the ammonia selective reduction-type NOx catalyst. In this case, the urea water is injected into the exhaust gas by using a urea-water injector or the like inserted in the exhaust passage. The urea in the atomized urea water injected into the exhaust gas from the urea-water injector is hydrolyzed by heat of the exhaust gas, and ammonia produced as a result is supplied to the ammonia selective reduction-type NOx catalyst. The ammonia is once adsorbed by the ammonia selective reduction-type NOx catalyst, and as the denitration reaction between the ammonia and the NOx in the exhaust gas is promoted by the ammonia selective reduction-type NOx catalyst, the NOx is reduced to purify the exhaust gas.
In the case of such an exhaust purification device provided with the ammonia selective reduction-type NOx catalyst, immediately after the engine is started, the temperature of the ammonia selective reduction-type NOx catalyst and of the urea-water injector is low, and thus the urea in the injected urea water cannot be satisfactorily hydrolyzed within the exhaust gas. For this reason, the injection of the urea water from the urea-water injector needs to be kept stopped until the exhaust temperature rises to a certain temperature after the start of the engine.
However, if the supply of the urea water is stopped after the start of the engine until the temperature of the ammonia selective reduction-type NOx catalyst and of the urea water injector rises to a certain level, the selective reduction of NOx by means of the ammonia selective reduction-type NOx catalyst using, as a reducing agent, the ammonia produced from the urea water is not carried out while the supply of the urea water is stopped, giving rise to a problem that the exhaust gas cannot be purified.
In the case of the series-type hybrid electric vehicle in particular, the engine is repeatedly started and stopped depending on the storage rate of the battery, as stated above. Each time the engine is started, therefore, a situation is repeated where the exhaust gas fails to be purified by selectively reducing NOx. As a result, the exhaust purification efficiency of the ammonia selective reduction-type NOx catalyst significantly lowers, posing a major problem.